1. Field of the Invention
This invention relates to a tolan derivative as a novel compound and a liquid crystal composition containing the same.
2. Description of the Related Art
Display elements making use of liquid crystals utilize the optical anisotropy and dielectric anisotropy of liquid crystal substances, and include those . of various modes such as T-N (twisted nematic) type, DS (dynamic scattering) type, guest-host type, DAP type, etc. depending on their display modes. Properties required for liquid crystal substances used according to these modes each vary, but common properties thereto consist in that the substances exhibit liquid crystal phases in a temperature range as broad as possible and are stable to moisture, heat, air, etc. At present, however, there is no single compound which satisfies all of such requirements; hence it is the present status that liquid crystal compositions obtained by mixing several kinds of liquid crystal compounds or mixing similar compounds to liquid crystals with several kinds of liquid crystals have been used.
As examples of tolan derivatives used as a component of liquid crystal materials, compounds expressed by the following formulas are respectively disclosed in (1) Japanese patent application laid-open No. Sho 6055142/1985 and (2) Japanese patent application laidopen No. Sho 61-260031/1986: ##STR2## wherein R represents an alkyl group and X represents a halogen atom; and ##STR3## wherein R represents an alkyl group; X.sub.1 represents hydrogen atom or a halogen atom; and X.sub.2 represents a halogen atom.
These tolan compounds are liquid crystal materials characterized by having halogen atom(s) as a substituent and a large optical anisotropy value (hereinafter abbreviated to .DELTA.n).
Among these compounds, the compound (1) has a relativelyhigh clearing point, but since its compatibility with other liquid crystal compounds is inferior, crystals are deposited; hence the compound has a drawback that it is impossible to make use of its specific feature. The compound (2) has no liquid crystal phase and hence when it is used as a component of liquid crystal compositions, it has a drawback that it reduces N-I point of the compositions too much.